Aqueous solutions of crude terephthalic acid are purified by hydrogenation in the presence of plural noble metal catalysts in separate layers. Initially, the solution to be purified is passed through a layer of ruthenium-on-carbon catalyst, rhodium-on-carbon catalyst, or platinum-on-carbon catalyst, and thereafter through a layer of palladium-on-carbon catalyst. Optionally, the purified aqueous terephthalic acid solution can be treated further to decrease the 4-carboxybenzaldehyde content thereof by passing it through a layer of rhodium-on-carbon catalyst downstream from the palladium-on-carbon catalyst layer. |